Girls playing on mixed vs. girls' hockey teams?

My 6-year-old daughter plays hockey on a mixed Mite team. Next year we will likely be pressured to sign her up for the new U8 girls' team. I think she would develop better as a hockey player if she plays with the boys. She's very competitive and is one of the best skaters. Additionally, it will cost about $500 to play on a Mite team vs. $1,200 to play on the U8 girls' team (few players to split the cost of ice time). At this point she doesn't mind playing with boys; she doesn't really know the difference. Anyone go through this? Can a hockey association require girls to play on a girls team if one exists? I've spoken with a few Division I hockey players who said to keep her in the co-ed ranks as long as possible (until the boys get too big and checking is a concern). I'd appreciate feedback on this, especially from anyone who has gone through this.